Iraqi forces have begun their assault on Mosul's Old City as they press their battle to retake the the rest of the northern city from the Islamic State of Iraq and the Levant (ISIL) group.
State TV has aired live footage showing thick black smoke rising from the Old City and gunfire echoing from inside.
It said leaflets were distributed urging civilians to leave through five "safe corridors".
"The army, counterterrorism forces and federal police launched an attack on the Old City," Staff Lieutenant-General Abdulamir Yarallah, who commands operations in Iraq's Ninevah province, said in a statement on Sunday.
Staff Lieutenant-General Abdulwahab al-Saadi, a senior commander with the Iraqi Counter-Terrorism Service, told state TV he expects ISIL to put up a "vicious and tough fight".
Overnight air strikes
Iraqi forces backed by the air strikes of the US-led coalition are pressing a months-long offensive to retake the Old City on the west side of the city from ISIL, also known as ISIS.
The US-led coalition providing air support to the Iraqi forces on the ground launched at least 20 air strikes overnight on targets in the Old City in advance of Sunday's push, said Al Jazeera's Bernard Smith, reporting from the nearby city of Erbil.
"Not long after that, the Iraqi special forces, the regular army and the federal police from the north and the south of the Old City launched [this offensive] to retake this last part of Mosul that remains under ISIL control," he said.
Taking back the Old City, a densely populated warren of narrow alleyways on the western side of Mosul, is crucial to recapturing the whole of the former ISIL bastion.
The UN says around 150,000 civilians are trapped in the ancient city centre along with hundreds of ISIL fighters.
Bruno Geddo, the UN refugee agency's representative in Iraq, said ISIL had been capturing civilians and forcing them into the Old City.
"We know that ISIS moved them with them as they left ... locations where the fighting was going on," Geddo said in Geneva, using a different acronym for ISIL.
"These civilians are basically held as human shields in the Old City."
With virtually no food, water or electricity left in the area, the civilians are "living in an increasingly worsening situation of penury and panic", he said.
"They are surrounded by fighting on every side."
Earlier this month, the global body warned of a "significant escalation" of civilian deaths in the city.
Al Jazeera's Smith said the push to clear the last part of Mosul would not be easy.
"We're talking about a very concentrated area ... narrow alleyways and streets, house-to-house fighting ... . It's going to be a very challenging battle for the Iraqi security forces," he said.
Since the battle to retake Mosul began nine months ago, an estimated 862,000 people have been displaced from the city, although 195,000 have since returned, mainly to the liberated east of the city.
Iraqi forces launched the battle for Mosul in October, retaking the eastern part of the city in January and starting the operation for its western part the next month.
The Old City is home to the centuries-old al-Nuri Mosque, where ISIL leader Abu Bakr al-Baghdadi delivered a Friday sermon in 2014 as his group declared an Islamic caliphate in the areas it controlled in Syria and Iraq.
ISIL has lost much of that territory over the last three years, and Mosul is their last urban bastion in Iraq.